Recently there has been considerable interest within the detergents industry in the production of detergent powders having relatively high bulk density, for example 600 g/liter and above.
Generally speaking, there are two main types of processes by which detergent powders can be prepared. The first type of process involves spray-drying an aqueous detergent slurry in a spray-drying tower. In the second type of process the various components are dry-mixed and optionally agglomerated with liquids, e.g. nonionics.
The most important factor which governs the bulk density of a detergent powder is the bulk density of the starting materials in the case of a dry-mixing process, or the chemical composition of the slurry in the case of a spray-drying process. Both factors can only be varied within a limited range. For example, one can increase the bulk density of a dry-mixed powder by increasing its content of the relatively dense sodium sulphate, but the latter does not contribute to the detergency of the powder, so that its overall properties as a washing powder will generally be adversely affected.
Therefore, a substantial bulk density increase can only be achieved by additional processing steps which lead to a densification of the detergent powders. There are several processes known in the art leading to such densification. Particular attention has thereby been paid to the densification of spray-dried powders by post-tower treatment.
The European patent application 219,328 (UNILEVER) discloses a granular low-phosphate detergent composition prepared by spray-drying a slurry to give a base powder containing a low to moderate level of sodium tripoly-phosphate builder and low levels of inorganic salts, and then post-dosing solid material including sodium sulphate of high bulk density and of smaller particle size than the base powder, thus filling the voids between the base powder particles and producing a product of high bulk density.
The Japanese patent application 61 069897 (KAO) discloses a process in which a spray-dried detergent powder containing a high level of anionic surfactant and a low level of builder (zeolite) is subjected successively to pulverizing and granulating treatments in a high-speed mixer/granulator, the granulation being carried out in the presence of an "agent for improving surface properties" and optionally a binder. It would appear that in the high-speed mixer/granulator, the spray-dried powder is initially broken down to a fine state of division; the surface-improving agent and optional binder are then added and the pulverized material granulated to form a final product of high bulk density. The surface-improving agent, which is a finely divided particulate solid such as fine sodium aluminosilicate, is apparently required in order to prevent the composition from being formed into large balls or cakes.
The process described in this Japanese patent application is essentially a batch process and is therefore less suitable for the large scale production of detergent powders.
The European patent application 229,671 (KAO) discloses post-dosing a crystalline alkaline inorganic salt, for example sodium carbonate, to a spray-dried base powder prepared as in the above-mentioned Japanese application 61 069897 (KAO) and containing a restricted level of water-soluble crystalline inorganic salts, to produce a high bulk density product.
The British patent application 1,517,713 (UNILEVER) discloses a batch process in which spray-dried or granulated detergent powders containing sodium tripolyphosphate and sodium sulphate are densified and spheronized in a "marumerizer" (Trade Mark). This apparatus comprises a substantially horizontal, roughened, rotatable table positioned within, and at the base of, a substantially vertical, smooth-walled cylinder.
The British patent application 1,453,697 (UNILEVER) discloses the use of a "marumarizer" (Trade Mark) for granulating together detergent powder components in the presence of a liquid binder to form a granular detergent composition.
The disadvantage associated with this apparatus is that it produces powders or granules having a rather wide particle size distribution, and in particular containing a relatively high proportion of oversize particles. Such products exhibit poor dissolution and dispersion characteristics, particularly in low-temperature short duration machine washes as used in Japanese and other far-eastern washing machines. This can be apparent to the consumer as deposits on washed fabrics, and in machine washing leads to a high level of wastage.
The European patent application 220,024 (Procter & Gamble) discloses a process in which a spray-dried detergent powder containing a high level (30-85% by weight) of anionic surfactant is mixed with an inorganic builder (sodium tripolyphosphate, or sodium aluminosilicate and sodium carbonate) and compacted under high pressure using a roll compactor ("chilsonator"); the compacted material, after removal of oversize material and fines, is then granulated using conventional apparatus, for example a fluidized bed, tumble mixer, or rotating drum or pan.
In an article in Seifen-Ole-Fette-Wachse (114, 8, pages 315-316 (1988)), B. Ziolkowsky describes a process for obtaining a detergent powder having an increased bulk density by treating a spray-dried detergent composition in two-step post-tower process, which can be carried out in a Patterson-Kelly Zig-Zag.RTM. agglomeration apparatus. In the first part of this machine, the spray-dried powder is fed into a rotating drum, in which a liquid-dispersing wheel equipped with cutting blades is rotating. In this first processing step a liquid is sprayed on to the powder and is thoroughly admixed therewith. By the action of the cutters, the powder is pulverized and the liquid causes agglomeration of the pulverized powder to form particles having an increased bulk density compared to that of the starting material.
The bulk density increase obtained is dependent on a number of factors, such as the residence time in the drum, its rotational speed and the number of cutting blades. After a short residence time, a light product is obtained, and after a long residence time a denser product.
In the second part of the machine, which is essentially a rotating V-shaped tube, the final agglomeration and conditioning of the powder take place. After the densification process, the detergent powder is cooled and/or dried.
Although it is possible by means of one or more of the above-mentioned processes to prepare detergent powders having a high bulk density, each of these routes has its specific disadvantages. It is therefore an object of the present invention to provide an improved continuous process for obtaining high bulk density granular detergent compositions or components thereof, having a bulk density of at least 650 g/l. The process should be especially suitable for the large scale manufacture of such compositions.
We have now found that the above and other objects can be achieved by the process of the present invention. According to the invention, it was found that a substantial increase of the bulk density of a detergent powder can only be obtained if the particle porosity, which may be in the order of 20-70% for a spray-dried base powder, is successfully reduced to, or kept at, values of less than 10%, preferably less than 5%. This can be achieved by carrying out the detergent powder manufacturing process under conditions wherein a particulate starting material is brought into or maintained in a deformable state.